1. Field of the Invention
The present invention relates to an infrared sensor that detects infrared radiation from a measurement object to thereby measure the temperature or the like of the measurement object.
2. Description of the Related Art
When the heat generation state of an electronic component (device) on a circuit substrate, such as a switching element such as FET, an electrolytic capacitor, or the like, is detected, there has been known a method in which a temperature sensor is provided on a circuit substrate in the vicinity of the device or a temperature sensor is provided in a heat sink connected to the device so as to indirectly know an element temperature from the temperature of the circuit substrate or the heat sink. In the method, the temperature of a measurement object such as a device or the like is indirectly detected, resulting in an increase in detection errors and difficulty in highly accurate detection.
Conventionally, infrared sensors have been used as temperature sensors that detect infrared radiation emitted from a measurement object without contact to thereby measure the temperature of the measurement object.
For example, Patent Document 1 discloses an infrared sensor that is a temperature sensor for measuring the temperature of a heating fixing roller or the like used in a fixing device for a copier and that includes a resin film that is disposed on a holder, a heat sensitive element for infrared detection that is provided on the resin film and detects infrared radiation via a light guide unit of the holder, and a heat sensitive element for temperature compensation that is provided on the resin film in a light-shielding state and detects the temperature of the holder. In the infrared sensor, an infrared absorbing film is formed on the inside surface of the light guide unit and an infrared absorbing material such as carbon black or the like is contained in the resin film so as to increase infrared absorption. Also, in the infrared sensor, the heat sensitive element is built into the holder which is a housing of a substantially block shape formed from a metal material such as aluminum having high heat conductivity and low thermal emissivity.
Also, Patent Document 2 discloses an infrared detector including a heat sensitive element for infrared detection, a heat sensitive element for temperature compensation, a resin film on which the heat sensitive element for infrared detection and the heat sensitive element for temperature compensation are fixed in a tight contact manner, and a case having a frame body in which the heat sensitive element for infrared detection is arranged on the side of an entrance window for infrared radiation and the heat sensitive element for temperature compensation is arranged on the side of a shielding unit for shielding infrared radiation. In the infrared detector, an infrared absorbing material such as carbon black or the like is contained in the resin film so as to increase infrared absorption and the frame body is formed with a material having excellent heat conductivity in order to eliminate the thermal gradient between the heat sensitive element for infrared detection and the heat sensitive element for temperature compensation. For the heat sensitive element for infrared detection and the heat sensitive element for temperature compensation, a radial lead thermistor in which a lead wire is in connection with a thermistor is employed. Furthermore, in the infrared detector, heat sensitive elements are built into the case formed of a resin or a metal.
These infrared sensors disclosed in Patent Documents 1 and 2 employ a structure in which an infrared absorbing material such as carbon black or the like is contained in the resin film and one of the heat sensitive elements is shielded from light so as to provide temperature compensation. However, the resin film containing an infrared absorbing material exhibits high heat conductivity and thus it becomes difficult to cause the temperature difference between the heat sensitive element for infrared detection and the heat sensitive element for temperature compensation. In addition, in order to increase the temperature difference between these heat sensitive elements, the distance between these heat sensitive elements needs to be spaced apart from each other, resulting in an increase in the overall shape of the infrared sensor and a difficulty in achieving size reduction of the same. Furthermore, the structure for shielding the heat sensitive element for temperature compensation from light needs to be provided on the case itself, which makes the infrared sensor more expensive.
In Patent Document 2, the frame body having excellent heat conductivity is employed, and thus, heat supplied from the infrared absorbing film is also dissipated, resulting in degradation in sensitivity. In addition, since a radial lead thermistor in which a lead wire is in connection with a thermistor is employed, heat conduction through a space occurs between the thermistor and the lead wire.
Furthermore, although the infrared sensor disclosed in Patent Document 2 employs a structure in which one of the heat sensitive elements is shielded from infrared radiation with the housing, the shielding portion of the case which only blocks infrared radiation absorbs infrared radiation, resulting in changes in the temperature of the shielding portion. Consequently, the heat sensitive element for temperature compensation is inappropriate for use as a reference.
Thus, as disclosed in Patent Document 3, there has been developed an infrared sensor that includes an insulating film, a first heat sensitive element and a second heat sensitive element that are disposed on one surface of the insulating film so as to be separated apart from one another, a plurality pairs of conductive wiring films that are formed on one surface of the an insulating film and are separately connected to the first heat sensitive element and the second heat sensitive element, an infrared absorbing film that is disposed on the other surface of the insulating film so as to face the first heat sensitive element, and an infrared reflection film that is disposed on the other surface of the insulating film so as to face the second heat sensitive element.
In the infrared sensor, a portion at which the infrared absorbing film is provided absorbs infrared radiation and a portion at which the infrared reflection film is provided reflects infrared radiation, so that a favorable temperature difference can be obtained between the first heat sensitive element and the second heat sensitive element on a thin insulating film having low heat conductivity. Specifically, even in the case of the insulating film having low heat conductivity without containing an infrared absorbing material or the like, the heat generated by infrared absorption can be conducted only to a portion immediately above the first heat sensitive element on the insulating film. In particular, the heat conduction from the infrared absorbing film is performed through the thin insulating film, resulting in no degradation in sensitivity and obtaining high responsiveness. Since the area of the infrared absorbing film can be arbitrarily set, a viewing angle for infrared detection can be set by area in accordance with the distance to the measurement object, resulting in obtaining high light receiving efficiency. Also, the infrared reflection film reflects infrared radiation directed toward a portion immediately above the second heat sensitive element on the insulating film so that infrared absorption can be prevented. It should be noted that since the infrared absorbing film and the infrared reflection film are formed on the insulating film, a medium of heat conduction between the infrared absorbing film and the infrared reflection film is only the insulating film on which these films are opposed to one another other than air, resulting in a reduction in cross-sectional area for heat conduction. Thus, heat is difficult to be conducted to the heat sensitive elements which are opposed to one another so that heat interference is reduced, resulting in an improvement in detection sensitivity. As described above, the infrared sensor has a structure in which the first heat sensitive element and the second heat sensitive element, for which the effects of heat are suppressed from one another, on the insulating film having low heat conductivity, measure the partial temperature of the insulating film directly below the infrared absorbing film and directly below the infrared reflection film, respectively. Thus, a favorable temperature difference can be obtained between the first heat sensitive element for infrared detection and the second heat sensitive element for temperature compensation, resulting in an increase in sensitivity.